An example of a directional coupler which detects an output of an RF circuit module reliably and accurately is disclosed in Japanese Patent Application Laid-Open Publication No. 2002-43813 (Patent Document 1). In this example, the directional coupler that detects the output of the RF circuit module has a structure in which a main-line and a sub-line overlap each other via a dielectric. And, the width of the main-line is narrower than the width of the sub-line, and both side edges of the main-line are positioned inside of both side edges of the sub-line so that the entire width of the main-line faces the sub-line certainly.
And, an example of a small, high-performance coupler with excellent directivity, small insertion loss, and small deterioration in a reflection characteristic is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-133817 (Patent Document 2). In this example, the main-line and the sub-line are arranged so that at least parts of the main-line and the sub-line are approximately parallel with each other in their side surfaces, and therefore, in a side-edge-type directional coupler in which a main-line and a sub-line are coupled in distributed-constant-type, a length of the sub-line is longer than a length of the main-line. And, the main-line is formed of a line in an approximately straight-line shape or a line in an approximately straight-line shape bended at a predetermined position, and has a structure not wound in a spiral fashion. The sub-line is formed of a line in an approximately straight-line shape bended at a predetermined position, and has a structure wound in a spiral fashion.
And, an example of a directional coupler with no deterioration in line impedance of the main-line and the sub-line even with downsizing is disclosed in Japanese Patent Application Laid-Open Publication No. 11-284413 (Patent Document 3). In this example, the main-line composed of a swirling pattern is formed in one layer over a substrate provided with a ground electrode, and a sub-line composed of a swirling pattern is formed in one layer positioned on an upper layer of the layer via an insulating film.